Centrifugal pump of the free surface type



R. W. ATZ

March 11, 1969 CENTRIFUGAL PUMP OF THE FREE SURFACE TYPE Filed July 14,1967 INVENTOR. ROBERT W ATZ BY ,x 4 QM ATTORNEY United States Patent 3Claims ABSTRACT OF THE DISCLOSURE A centrifugal pump of a free-surfacedtype is described wherein a portion of the fluid flow recirculateswithin the pump in the presence of a covering gas. A pair of auxiliaryimpellers are utilized to control the liquid level within the pump whilealso preventing flooding of the pump and providing the recirculatingfluid therein, for maintaining fluid pressure at a pressurized bearing.

Background of the invention:

The invention relates to a centrifugal pump and more particularlyrelates to such a pump of the free-surface type. This invention arose inthe course of or under Contract AT(11-l)-GEN8.

Centrifugal pumps are known in the art. One such type of pump isfrequently referred to as the free surface type. Such free-surface pumpsare frequently used in the nuclear reactor art for pumping liquid metalcoolant. Such freesurface type pumps are characterized by the fact thatthey have no internal seal separating the impeller from the othercomponent parts Within the pump casing. Acc0rd ingly, in such avertically oriented pump the fluid rises vertically beyond the level ofthe impeller; therefore the prior art pumps include a means which isnecessary for controlling the level of the fluid in the pump as well asfor removing this fluid. Usually an overflow line and internal drainsare provided for returning the fluid back to the inlet of the pump. Thedesign of such pumps necessitates certain limitations on the use of thepump as to its position in the fluid system which not only requiresadditional components but also limits the usefulness and elliciency ofthe pump. Since oxygen must normally be excluded from any system havingliquid metal such as sodium, an inert covering gas, usually of nitrogenor argon, is used in liquid metal pumps.

Summary of the invention An object of the invention is to provide animproved free-surface type pump wherein overflow lines are eliminated.

It is an object of this invention to provide a freesurface type pumpcapable of being used at low points in the piping system whereby largesuction pressure is available at the pump.

It is another object of the invention to provide a freesurface type pumpcapable of operating at high shaft speeds with less danger ofcavitation.

A further object of this invention is to provide a freesurface type pumpwherein flooding of the pump is prevented by the use of auxiliaryimpellers to create a pressure greater than the pressure created by themain impeller and of sufiicient size to provide a fluid flow greaterthan the leakage in the pump.

Another object is to provide a pump having a pressur- "ice The inventionprovides a centrifugal pump of the freesurface type wherein fluid ispermitted to leak beyond the main impeller into the main pump casing. Apressurized bearing region is provided in the casing to absorb radialloads. Leakage fluid builds up or fills a substantial part of the pumpcasing including the pressure bearing. Auxiliary impellers provide aback pressure on the fluid within the pump casing at a pressure greaterthan that of the main impeller to recirculate fluid throughout the pumpbearing.

Description of the drawing The figure shows the cross-sectionelevational view of a preferred embodiment of the invention.

Description 0 the invention In the figure the operation of the pump isas follows:

The incoming pump fluid is admitted to the inlet nozzle 1. The fluid ispumped by means of the centrifugal force created by rotation of theimpeller 2 and the fluid exits into the volute or diffuser 3 forsupplying a nuclear reactor system (not shown). The impeller is attachedto shaft 13 for rotation therewith from an external motor or otherdriving source. A significant feature of this freesurface type pump isthat essentially there is no permanent or complete seal between the mainimpeller and the other components of the pump; accordingly, the fluid atthe inlet is permitted to rise within the pump casing 5 to a leveldependent upon the pump and gas pressure and other factors such as theoverflow lines in the prior art pumps.

The discharge fluid in the volute 3 is at the discharge pressure createdby the main impeller 2 and this fluid passes out the discharge nozzle tothe nuclear reactor or heat exchanger as the case may be. A portion ofthis high pressure fluid leaks through the labyrinth seal 4 which formsthe alignment fit between the pump casing and its internal componentparts. The leakage fluid from seal 4 collects in the pump casing 5 alongwith any other leakage fluid existing because of the absence of anypermanent seal between the impeller housing 18 and pump casing 5.

High pressure fluid is delivered to the pressurized bearing 6 throughthe bearing supply line 7 formed in the impeller housing. Recirculatoryflow ports 8 serve to return a part of the efl luent from thepressurized bearing 6 to inlet side of the impeller; the balance of theleakage from the high pressure bearing passes out the bearing flow port9 and collects in the pump casing 5 causing the fluid to rise in levelwithin the casing. The fluid rises in the casing and along the rotorshaft until the fluid level reaches the inlet to the upper side of thetop auxiliary impeller 10.

The liquid is pumped by the top and bottom auxiliary impellers 10 and 11respectively to the chamber 19 supplying fluid to the pressurizedbearing through the collected fluid return lines 12. Since theseauxiliary impellers 10 and 11 pump only liquid rather than gas againstthe pump discharge pressure, there is no danger of driving gas into thepressurized bearing.

It should be noted that the pressurized chamber 19 receives highpressure liquid flow from both the main and auxiliary impellers. Thefluid passes through opening 20 into the bearing along the shaft and thepump casing to the top of the auxiliary impellers.

It is understood in the liquid metal art, that when sodium is in contactwith oxygen or air, a violent reaction usually occurs; accordingly, suchsodium pump systems usually use a cover gas within the pump casing inthe form of nitrogen or inert gas such as argon to prevent suchaccidents. Such a gas is usually present in the upper half of the pumpcasing, that is above the auxiliary impellers.

The auxiliary impellers 10 and 11 in this invention are sized andfunction to deliver slightly more fluid than can be supplied by theleakage from the labyrinth seal 4 and the pressurized bearing; and theyoperate in series to de liver a back pressure at the pressure bearinggreater than that produced by the pumps main impeller 2. These impellersreceive most of their fluid through the supply ports 21.

The pump shaft 13 is supported at the floor plate 114 by a thrustbearing and gas seal assembly 15; thermal protection for this upper pumpregion is provided by the thermal heat barrier cavity 16 which mayinclude air or conventional insulation and the radiation heat barrier17.

Drain ports 28 are provided for removing the liquid from the pumpcasing, when the pump is to be cleaned and aid in reducing the amount offluid to be pumped by auxiliary impellers and 11.

The significant improvement in this invention is that it eliminates theoverflow lines required for prior art freesurface type pumps and it isalso possible to discard the gas-vent lines in such prior art pumps. Itis, therefore, possible to place this pump at the lowest surface in apiping system since the level of the free liquid surface is determinedby the position of the top auxiliary impeller. Furthermore, bymaintaining the proper gas pressure in the pump casing it is possible toprotect the pump against flooding.

The advantages of this pump are clearly indicated by the fact that thispump can be placed at the lowest point or at a low point in a fluidpiping system such that the amount of net positive suction pressureavailable to the pump inlet is maximized. This condition permitsoperating the pump at higher shaft speeds with less danger of cavitationand will also permit using a smaller and less costly unit.

The use of the auxiliary impellers to handle the leakage within the pumpis certainly an important aspect of this invention such that theycontrol the level of the liquid surface in the pump. In addition, theauxiliary impellers are designed to create a back pressure on the liquidgreater than that created by the mainimpeller to provide a recirculatingback flow of liquid to the pressure bearing to exceed the leakage intothe main pump body from the main impeller; thus, it is clearly seen thatthese auxiliary impellers provide an important function in thisinvention. While we have shown one preferred embodiment of the inventionas required, it will be obvious that there are numerous modifications ofthe individual components of this invention; accordingly, the scope ofthe invention is defined in the following claims.

I claim:

1. A centrifugal pump of the free surface type comprising a pump casingdefining a fluid discharge port, an impeller mounted for rotation withinsaid casing, said impeller and easing providing a leakage flow paththerebetween for providing a free-surface liquid level within the pumpcasing, a pressurized bearing associated with said impeller and adaptedfor receiving a portion of said leakage flow, a bearing supply lineconnected between said fluid discharge port and said pressurizedbearing, means including two auxiliary impellers within said casing andmeans including a fluid flow port for connecting the pressurized bearingto said auxiliary impeller means for creating a recirculating fluid flowtherein for supplying said pressurized bearing with liquid metal flow,and fluid return mains connected for returning fluid from auxiliaryimpellers to the pressurized bearing.

2. Apparatus as in claim 11 and further including an internal drainmeans connecting the fluid port means to the main impeller.

3. A centrifugal pump of the free-surface type comprising an elongatedpump casing,

a pump housing attached to one end of said casing and including an inletand outlet ports,

a rotatable shaft extending through said casing,

an impeller within said housing adapted to pump fluid from the inlet tothe outlet when rotated by said shaft,

a pressurized bearing cooperating with the shaft,

an annular pressurized chamber formed between the bearing and easing,said chamber adapted to supply liquid to the bearing surface with theshaft,

a bearing supply line connected between the output port and one end ofsaid chamber for supplying high pressure fluid from the pump housing,

an auxiliary impeller aflixed to the shaft within the casing adapted forpumping fluid accumulated within the casing at a pressure greater thanthe pressure at the output port,

a fluid line communicating with the other end of the pressure chamberfor returning fluid to the pressure chamber under high pressure createdby the auxiliary impeller,

a drain port connected for returning fluid from the casing to the pumphousing,

and a recirculatory flow port connected for returning fluid from withinthe casing to the inlet port of the pump whereby bidirectional fluidflow paths are provided between the casing and pump housing andrecirculation of fluid with the casing and the pressurized bearing isprovided.

References Cited UNITED STATES PATENTS 2,796,027 6/1957 Brown 103-1122,811,109 10/1957 Brill et al. 2,959,133 11/1960 Erwin. 3,118,384 1/1964Sence et al. 1,651,881 12/1927 Frickey et al.

HENRY F. RADUAZO, Primary Examiner.

US. Cl. X.R. 103112

